Articles of footwear, such as shoes, boots, slippers, sandals, and the like, are generally composed of two primary elements: an upper for securing the footwear to a user's foot; and a sole structure for providing subjacent support for the foot. Uppers may be fabricated from a variety of materials—including textiles, foams, polymers, natural and synthetic leathers, etc.—that are stitched or adhesively bonded together to form a shell or harness for securely receiving a foot. For sandals and slippers, the upper may have an open toe or heel construction or may be generally limited to a series of straps extending over the instep and, in some designs, around the ankle. Conversely, boot and shoe designs incorporate a full upper with a closed toe or heel construction and an ankle opening through a rear quarter portion that provides access to the footwear's interior, facilitating entry and removal of the foot into and from the upper. A shoelace or strap may be utilized to secure the foot within the upper.
The sole structure is generally attached to a lower portion of the upper, positioned between the user's foot and the ground. In many articles of footwear, including athletic shoes, the sole structure is a layered construction that generally incorporates a comfort-enhancing insole, an impact-mitigating midsole, and a surface-contacting outsole. The insole, which may be located partially or entirely within the upper, is a thin and compressible member that provides a contact surface for the underside of the user's foot. By comparison, the midsole is mounted underneath the insole, forming a middle layer of the sole structure. In addition to attenuating ground reaction forces, the midsole may help to control foot motion and impart stability. Secured to the underside of the midsole is an outsole that forms the ground-contacting portion of the footwear and is usually fashioned from a durable and wear-resistant material that includes features for improving traction.